Conventional techniques relevant to a pressure sensitive capacitance element are described in U.S. Pat. No. 6,631,645 that presents an electrostatic capacitance type pressure gauge manufactured through etching of a sacrificial layer, such as shown in FIG. 1. This pressure gauge is constituted of a pressure sensitive capacitance element 2 whose capacitance value changes with an applied pressure, and a reference capacitance element 3 although whose capacitance value is similar to that of the capacitance element 2 it does not change with the applied pressure. The structure and operation principle of this pressure gauge is also described in the said patent.
FIG. 1 is a cross sectional view of the prior art semiconductor pressure sensor gauge. On the surface of a silicon substrate 1, a pressure sensitive capacitance element 2, a reference capacitance element 3 and a capacitance-voltage conversion circuit 4 made of C-MOS are formed.
The pressure sensitive capacitance element 2 has a fixed electrode 6 embedded in the upper surface layer of the substrate, and a movable electrode 5. These electrodes are disposed faced each other via a small space 7 to constitute a capacitance element structure. Formed on the variable electrode 5 are a sealing film 9 for vacuum sealing the inside of the small space 7 and a shielding and surface passivation film 10 formed on the sealing film 9. The structure of the reference capacitance element 3 is similar to that of the pressure sensitive capacitance element 2. However, in place of the variable electrode 5, a fixed electrode 8 with support columns is used to constitute the capacitance element whose capacitance will not change with an applied pressure. The diameter of the fixed electrode of the reference capacitance element 3 is made longer than that of the fixed electrode of the pressure sensitive capacitance element 2.
FIG. 2 is a top plan see-through view of another prior art capacitance-type pressure sensor described in U.S. Pat. No. 6,640,642 and FIG. 3 is a sectional view of the same taken along a line A—A in FIG. 2. Referring to FIGS. 2 and 3, reference numeral 20 denotes a semiconductor substrate having one surface over which a silicon oxide film 12 is deposited as an insulation film. Disposed on the silicon oxide film 2 in a matrix-like pattern are n-type polysilicon films 13 which serve as electrodes (fixed electrodes), respectively, wherein the n-type polysilicon films 13 are electrically connected in parallel with one another, as can be seen in FIG. 2. Further formed or deposited over the silicon oxide film 12 and the n-type polysilicon films 3 is a silicon nitride film 14 as an insulation film that is to serve as an etching stopper when a cavity region 18 is formed.
Disposed internally of the cavity region 18 are n-type polysilicon film portions 17 that serve as diaphragm fixing portions for supporting a sheet of diaphragm while partitioning regionally the diaphragm into a plurality of diaphragm sections each of a predetermined size for a plurality of sensor unit regions, respectively. Further, provided over the cavity region 18 is an n-type polysilicon film portion 16 which constitutes a part of the diaphragm and which serves as an electrode (movable electrode).
Deposited over the n-type polysilicon film portion 16 is a silicon oxide film 19 in such a manner that the cavity region 8 is thereby vacuum-sealed, i.e., sealed off in the evacuated state. Further, a fixed electrode lead-wire 15 and a movable electrode lead-wire 11 are provided for the fixed electrodes (n-type polysilicon film) 13 and the movable electrode (n-type polysilicon film portion) 16, respectively, wherein both the fixed electrodes and the movable electrode are lead out to be electrically connected to a capacitance detecting circuit (not shown).
Capacitive micromachined ultrasonic transducers have been emerging as an attractive alternative to piezoelectric transducers. They offer a larger set of parameters for optimization of transducer performance as well as ease of fabrication and electronic integration. The fabrication and operation of micromachined ultrasonic transducers have been described in many publications and patents. For example, U.S. Pat. Nos. 5,619,476; 5,870,351 and 5,894,452, incorporated herein by reference, describe the fabrication of capacitive-type ultrasonic transducers in which membranes are supported above a substrate by insulative supports such as silicon nitride, silicon oxide and polyamide. The supports engage the edges of each membrane. A voltage applied between the substrate and a conductive film on the surface of the membrane causes the membrane to vibrate and emit sound waves. The membranes can be sealed to provide operation of the transducers immersed in a liquid, as described in U.S. Pat. No. 6,493,288. The transducer may include a plurality of membranes of the same or different sizes and/or shapes. In operation, one or more multi-element transducers can be in arrays with the electrical excitation controlled to provide desired beam patterns.
Consider the traditional capacitor micromachined ultrasonic transducers (CMUT) described in U.S. Pat. Nos. 5,619,476, 5,870,351, 5,894,452 and 6,493,288. In particular, consider as a theoretical example a device made with 100 cells where each cell has a diameter of 200 μm, a gap of 0.5 μm, and a membrane thickness of 1 μm. Consider that, in this device, all the individual cells are sealed such that the gap of the capacitor is not open to the environment, and thus can survive humid, indeed even wet environments. When used as a receiver, using the traditional detection scheme, with a dc voltage of 30.745 volts applied across the cells with the output applied to an amplifier with the following characteristics: Rin=2 MΩ, Cin=1 pF, Vnoise=1.4 nV/Hz, and Inoise=0.01 pA/√Hz, the signal to noise ratio drops sharply for frequencies below 100 kHz. In other words, at frequencies lower than 100 KHz, a signal is received, but at much lower sensitivity.
Referring to FIG. 4, traditional micromachined capacitive ultrasonic sensors are made up of multiple small sealed, evacuated cells, each including a membrane 21 coated with a metal electrode 22. The membrane 21 is supported at its edges spaced from conductive base 23 by an insulating support 24. The interior volume 25 is evacuated. The geometry and the material of the membrane, and the surrounding medium determine the mechanical response of the microphone.
Indeed, the CMUT described in U.S. Pat. No. 6,493,288 provides a microphone that has wider operable frequency bandwidth in theory. However, such CMUT device is mainly invented to measure the acoustic signal whose pressure amplitude fluctuates in response to human voice or environmental noise. It addition, it only measures the relative pressure variation rather than the absolute pressure level for which a reference capacitor whose capacitance does not change with pressure must be provided.